The invention relates to testing and monitoring optical fibers used in communications, e.g., in the public telephone network.
The telephone network includes connections of users' telephones to a central office (often referred to as the subscriber loop and currently typically implemented by pairs of copper wires) and connections between central offices, needed when the two parties to a call are connected to different central offices. Optical fibers have been extensively used to provide communication between different central offices. Optical fibers have also been used to provide communication from a central office to a remote digital terminal (RDT), and currently copper wires are then used over a short distance to make the connections from the RDT to the user's telephones. RDTs might, e.g., be located at a business customer's premises or at a remote curbside location that serves a number of residences. With the desire to increase bandwidth and to decrease the investment in copper wire in the future, the use of optical fibers in the subscriber loop is expected to increase.
Owing to the differences between copper wires and optical fibers, the automatic telephone test systems currently used to test large numbers of copper telephone lines at high speed cannot be used to test optical fiber telephone lines. Takasugi, H. et al., "Design and Evaluation of Automatic Optical Fiber Operation Support System", International Wire & Cable Symposium Proceedings 1990, pp. 623-629, describes a system for testing optical fiber cable networks automatically by remote control. The system includes a test controller that is located at the central office and is connected to a fiber under test in the subscriber loop via a fiber selector and an optical branch module that includes an optical coupler to the fiber under test. Filter-embedded connectors are used between the fibers and the units at the remote ends of the fibers to provide filters that transmit operating signals at 1310 nm wavelength and reflect test light at 1550 nm wavelength. The test controller includes an optical time domain reflectometer (OTDR), an optical power meter (OPM) and a light source.